Hey Doug! First you would need to have more than one link going between your switches to even have the possibility of a loop. Secondly, while I can't guarantee it, I've never seen a loop form when using mismatched native VLAN's on my trunks (and no I don't design bad networks so stop looking at me like that ;) ;))
I like to think of this as the poor man's method for layer 2 bridging between two VLAN's, also known as "VLAN leaking". If you are really bored and have a few switches lying around you can lab this up and tell us what you find. While It's possible that some IOS versions will err-disable the ports the majority will just give you an error message. The error is generated via CDP since, since CDPv2 will pass the native VLAN information between the switches. Lab it up and see what you find! Thank you, Steve E. Di Bias | Network Engineer CCNP (R&S), CCNA (R&S/Security), FNCNE, BCNE, CE|H, CCA, MCSE, MCSA, MCTS, MCITP, A+, Net+ Valley Health System | www.valleyhealthsys.com Direct: 702-369-7594 | Mobile: 702-241-1801 Email: [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Douglas Koobs Sent: Monday, October 03, 2011 6:27 PM To: [email protected] Subject: Re: [OSL | CCIE_RS] How does a native VLAN mismatch create a loop? Thinking about this a bit more, wouldn't the two VLANs just re-converge and create a loop-free topology around one bridge? Certainly not optimal, but not a loop... On Mon, 2011-10-03 at 20:39 -0400, Douglas Koobs wrote: > I'm reading the SWITCH foundation learning guide, and it states that a > trunk with mismatched native VLANs can lead to a loop because BPDUs > are sent to the IEEE MAC address 0180.c200.0000 > > I think I understand at a very high level how this could happen: The > root bridge of one of the VLANs would be superior to the root bridge > of the other VLAN, and some of the switches would begin to converge > around the root bridge from the wrong VLAN. > > However, this situation is still fuzzy around the edges to me, which > usually means I'm missing something fundamental. Am I? > > Thanks! > > Doug _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com UHS Confidentiality Notice: This e-mail message, including any attachments, is for the sole use of the intended recipient (s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution of this information is prohibited. If this was sent to you in error, please notify the sender by reply e-mail and destroy all copies of the original message. _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
